1. Field of the Invention
The present invention relates to what is called a FIMS (Front-Opening Interface Mechanical Standard) system or a load port apparatus, which is used when wafers (which are objects to be processed) stored in the interior of an airtight transfer container called a pod are transferred between semiconductor processing apparatuses in a semiconductor manufacturing process etc. More specifically, the present invention relates to a load port apparatus characterized in its mapping system for detecting the presence/absence of wafers in the interior of a pod placed thereon and a what is called mapping method for detecting the presence/absence of objects to be processed such as wafers.
2. Description of the Related Art
A widely used method in semiconductor manufacturing processes is keeping a highly clean condition only in the interior of three spaces including the interior of processing apparatuses, the interior of a pod in which wafers are stored to enable transfer wafers between processing apparatuses, and a mini-environment (or small space) through which the wafers are transferred between the pod and each processing apparatus, thereby controlling the cleanliness throughout the process. The pod as such is a container composed of a body in which wafers are stored and that has an opening provided on one side thereof through which wafers are brought into/out of it, and a lid for closing the opening to seal the interior space of the pod. The mini-environment has an opening portion that can be opposed to the opening of the pod, and a second opening portion provided on the semiconductor processing apparatus side opposite to the opening portion.
The load port apparatus includes a member or a wall called “side base” that constitutes a partition on which the opening portion is provided, a door that closes the opening portion, a door driving mechanism that control the motion of the door, and a support table on which the pod is to be placed. The support table can support the pod in such a way that the opening of the pod and the opening portion are opposed to each other. The support table is adapted to move the lid toward and away from the door together with the pod. The door can hold the lid of the pod. The door is driven by the door drive mechanism to open/close the opening portion while holding the lid, to be retracted downwardly away from the space between the opening portion and the second opening portion, and to be inserted into the aforementioned space between the opening portion and the second opening portion. A robot is provided in the mini-environment. The robot can move into and retract from the interior of the pod through the opening portion and the opening of the pod and transfer wafers between the interior of the pod and the semiconductor processing apparatus through the opening portion, the opening of the pod, and the second opening portion. As disclosed in the publication of Japanese Patent No. 4246420, when bringing wafers into/out of the pod, what is called wafer mapping operation is performed. In the wafer mapping operation, a sensor is inserted into the pod and moved in the direction along which the wafers are arranged to detect the presence/absence of each of wafers arranged at different levels.
To maximize the number of processed substrates per one pod, in which 25 wafers are stored normally, an attempt to put an additional wafer at the highest level, which is originally intended to serve as a reserve, of the pod has been made. This leads to a small gap left between the wafer at the highest or top level and the top inner wall of the pod. When the highest level is used, it is necessary to insert a sensor into the pod at a position very close to the inner wall of the pod in order to detect the wafer at that level in the mapping operation. Therefore, it is necessary for the sensor to be inserted slowly to prevent the contact of the sensor with the inner wall. This might lead to an increase in the time taken by the process. In the case where the sensor is driven by a cylinder for operations of the sensor such as upward, downward, advancing and retracting motions, it is difficult to simply synchronize these operations, and it is accordingly difficult to achieve both enhancement in the operation accuracy to prevent the contact with the inner wall and reduction in the operation time.